In recent years, with the development of electrophotographic technology, there is also an increasing need for an electrically conductive member used in each electrophotographic process. Among others, it is required that a developing roller used in a developing process have not only a predetermined electric resistance value but also a variety of characteristics corresponding to a development mechanism.
Conventionally, as a development method in which a non-magnetic mono component developer is used as a developer (toner), known is a development method (impression development) in which a toner is provided via a developing roller on an image holding body such as a photoconductor drum on which an electrostatic latent image is held, and the toner is attached to the latent image on the image holding body, thereby visualizing the latent image. By this method, since a magnetic material is not needed, simplification and miniaturization of an apparatus becomes easy, as well as colorization of toner becomes easy. Since, in this development method, development is performed by adhering a toner on the latent image on the image holding body by contacting the developing roller on which a toner is held on an image holding body which holds an electrostatic latent image, the developing roller used for the development method needs to be formed of an elastic body having electrical conductivity.
FIG. 2 illustrates one example of the structure of a development apparatus using impression development. In the illustrated development apparatus, a developing roller 10 is arranged between a toner supplying roller 11 which supplies toner and a photoconductor drum 12 which holds an electrostatic latent image in a state in which the developing roller 10 is in contact with the photoconductor drum 12. Each of these rollers, the developing roller 10, the photoconductor drum 12 and the toner supplying roller 11 rotates in the arrow direction in the figure, and toner 13 is supplied on the surface of the developing roller 10 by the toner supplying roller 11. The supplied toner is adjusted in an uniform thin layer by a layer forming blade 14. The developing roller 10 in this state rotates while being in contact with the photoconductor drum 12, and thus the toner formed in a thin layer adheres to the latent image on the photoconductor drum 12 from the developing roller 10, whereby the latent image is visualized. Reference numeral 15 in the figure indicates a transfer unit, where a toner image is transferred to recording media such as a paper. Reference numeral 16 indicates a cleaning unit, by which toner remaining on the surface of the photoconductor drum 12 after the image is transferred is removed by a cleaning blade 17. In addition, reference numeral 18 indicates a charge roller which is in contact with the photoconductor drum 12 and which electrifies the photoconductor drum 12.
In this case, the developing roller 10 needs to rotate while surely maintaining a state in which the developing roller 10 is in close contact with the photoconductor drum 12; therefore, normally, those which have a basic structure in which an elastic layer composed of an electrically conductive rubber, high-molecular-weight elastomer, high-molecular-weight foam, or the like to which electrical conductivity is imparted by adding thereto a conductive agent has been formed on the outer periphery of a shaft composed of a material having a good electrical conductivity such as a metal, and which have, on the outer periphery of the basic structure, one or more coating layers in order to obtain a desired surface roughness, electrical conductivity, hardness or the like, are used.
As an improvement technique relating to such a developing roller, for example, Patent Document 1 discloses a method of examining a toner support in which, for a toner support composed of a shaft having an excellent electrical conductivity and a semiconducting layer formed on the outer periphery thereof, when a voltage of 8 kV is applied to a corona discharger which is arranged allowing 1 mm spacing between the surface of the toner support and the corona discharger and a corona discharge is generated to electrify the surface, the absolute value of the decay rate of the surface potential 0.1 second to 0.2 seconds after applying an electric charge is examined. Patent Document 2 discloses a technique of adjusting the relative permittivity, the surface resistance, and the volume resistance of an intermediate transfer belt used for forming an image such that the potential which is charged in the first primary transfer decays to ⅓ of the transfer potential or lower by the time when the belt comes to the position of the next primary transfer.